1. Field of the Invention
The present invention relates to optics. More specifically, the present invention relates to systems and methods for directing and correcting high-power beams of electromagnetic energy.
2. Description of the Related Art
Directed energy weapons and specifically high-energy laser (HEL) weapons are being considered for variety of military applications with respect to a variety of platforms, e.g., spaceborne, airborne and land based systems to name a few. These weapons generally involve the use of the laser or other source of a high-power beam to track and destroy a target. To achieve mission objectives, directed energy weapons must be accurately steered and optimally focused. Steering involves line-of-sight control while focusing, with respect to HEL weapons, involves wavefront error correction. Currently, wavefront error correction is typically achieved using adaptive optics. The current state of the art in laser beam control adaptive optics requires placing one or more deformable mirrors within the highest power portion of the beam path. The conventional deformable mirror is typically a large element with a thin face sheet and a number of piezoelectric actuators. Actuators are located behind the face sheet and are electrically driven to push and pull on the surface thereof to effect the deformation required to correct wavefront errors in an outgoing beam. The size of the active region of the deformable mirror must accommodate the full size of the high power laser beam in the high power Coudé path prior to expansion via an output telescope.
In addition, one or more fast steering mirrors, may be used to correct for tilt and direct the line-of-sight. A coarse gimbal may be employed to correct for line-of-sight errors as well. A plurality of wavefront sensors are typically employed along with an aperture sharing element (ASE). The ASE allows a single shared aperture to be advantageously used for both the low power sensors and the high power output laser beam, ensuring that the path through the atmosphere taken by the high power beam is the same as that taken by the wavefront sensor and that the correction applied to the shared atmospheric path is optimal for the high-power beam.
Unfortunately, the use of delicate optical devices in the path of a high-power beam is problematic. This is due to the fact that the high-power beam will heat and distort the optical element unless the element is actively cooled or has a coating with a very low optical absorption coefficient. The most durable coatings require a high temperature application process. Deformable mirrors are typically coated after the face sheet is bonded to the actuators, which limits the maximum temperature to which the deformable mirror assembly may be exposed without degrading the bond. Therefore, coatings may need to be applied at lower than optimal temperature using a less robust coating process, thereby reducing durability and/or increasing manufacturing cost.
In addition, conventional adaptive optics systems using deformable mirrors are limited in performance. Conventional deformable mirrors systems are limited with respect to the speed at which the mirror drive signals are computed and the reaction speed of the deformable mirror mechanism to correct for aberrations. There is also a limitation with respect to the number actuators that can be used. The number of actuators that may be used determines the resolution or “order” of the mirror. The stroke of the conventional deformable mirror is limited. “Stroke” relates to the amount of mirror surface deflection that may be achieved before either the piezoelectric actuators exceed their dynamic range or the face sheet begins to fail. Further, a conventional continuous face sheet deformable mirror cannot correct for a pathology in the spatial phase pattern, such as a branch point or an abrupt phase discontinuity. A branch point is a “singularity” in a deeply scintillated phase pattern caused by atmospheric turbulence over a long propagation path in which the phase monotonically increases around a zero amplitude point like a corkscrew, thereby requiring an abrupt 2π phase correction within the spatial phase pattern. Abrupt phase discontinuities may be caused by the optical discontinuities between segments of a multi-segment primary mirror.
In U.S. Pat. No. 5,694,408, issued Dec. 2, 1997, (the teachings of which are incorporated herein by reference). Bott, Rice, and Zediker appear to disclose a scheme which allows the deformable element to be placed in the low intensity region between a master oscillator and an array of fiber power amplifiers. The approach is to pre-distort the phase of the oscillator beamlets after separation in a distribution network and before injection into the fiber amplifier array, such that the pre-distortion corrects both the piston error between the individual fibers and optical aberrations in the atmosphere. However, this scheme is practical only with a coherently-combined array of single-mode fiber amplifiers, as each fiber channel is correctable in piston only, not high order. Also, this scheme is not applicable to multi-mode laser media such as large core fiber amplifiers or bulk media lasers as contemplated for weapon class HEL devices and may not be scaleable to high power levels due to random, high frequency phase noise caused by pump-induced temperature fluctuations within the fibers.
In U.S. Pat. No. 5,096,795, issued Feb. 25, 1992 (the teachings of which are incorporated herein by reference). O'Meara and Valley appear to disclose several related schemes for using a liquid crystal light valve (LCLV) in a self-correcting adaptive optics system. This approach, however, places the LCLV in the high power beam path and is therefore limited by the damage susceptibility of the liquid crystal material.
Accordingly, a need existed in the art for a fast, large-stroke, high spatial bandwidth or high order system or method for effecting wavefront correction of a high-power beam. The need was addressed by the teachings of U.S. Pat. No. 6,809,307 entitled System and Method for Effecting High-Power Beam Control with Adaptive Optics in Low Power Beam Path, issued Oct. 26, 2004 to Byren et al., the teachings of which are incorporated herein by reference. This system uses a nonlinear phase conjugate mirror at the end of a two-pass amplifier beamline within the high-energy laser device. Phase distortions, caused by the atmosphere and aerodynamic boundary layer (aircraft applications) are sensed by a wavefront sensor and these phase distortions are imposed directly on the wavefront of a laser master oscillator. The pre-distorted master oscillator beam is then amplified in a first pass through the high power amplifier beamline, where it picks up additional phase distortions due to thermal lensing within the gain elements. This amplified, distorted signal is phase conjugated such that the wavefront is reversed on a second path through the distorting amplifier media. After the second pass, the beam is amplified to full output power, the distortions from the amplifiers are corrected, and the conjugate of the phase error due to the atmosphere and boundary layer is imprinted on the beam. Now, when the high power beam propagates through the atmospheric and boundary-layer distortions, these distortions are compensated and a high intensity beam is delivered to the target of interest.
The drawback of this approach is that a two-pass phase conjugate amplifier beamline is required and use of a nonlinear optical phase conjugate mirror may not be advantageous for all high-power laser architectures envisioned for HEL weapon applications.
Hence, a need exists in the art for a more simple and efficient HEL design. Specifically, a need exists in the art for an HEL design that is smaller in size and lighter in weight and does not require nonlinear optical phase conjugation.